Particulate matter in air is often undesirable (for example, irritating to breathe, or interferes with the performance of equipment). Therefore, there is a need to remove some or all of the particulate matter from air and gas streams over extended periods of time. For example, HVAC (heating, ventilation and air conditioning) air, aircraft cabin ventilation, clean room ventilation, air to engines for motorized vehicles, or to power generation equipment, gas streams directed to gas turbines, and air streams to various combustion furnaces often include particulate material that needs to be constantly filtered or otherwise removed.
Efficiency is the propensity of filter media to trap, rather than pass, particulates. Typically, larger particles are captured at a higher efficiency than smaller particles, and several particle sizes may be used in a test of efficiency. In some applications, it may be desirable to remove small particles at a high efficiency. Filter media are placed in an air or gas stream and restrict the flow of that stream to some extent. This is usually measured as a pressure drop across the filter (from the upstream to the downstream side). It may be desirable to have a low pressure drop across the filter media. For example, lower pressure drop may result in less energy usage to operate an HVAC system. Typically, initial pressure drop (that is, the pressure drop of clean filter media) is used to determine the pressure drop performance of a filter media. Generally speaking, as the filter media efficiency increases, the pressure drop increases. Filter media may be designed to optimize performance in view of the efficiency/pressure drop trade-off.
There is a continuing need for additional filter designs that provide desired or improved filtering efficiencies and initial pressure drop.